The present invention relates to a voltage-controlled filter such as an active filter circuit and, more particularly, to a technology effectively used for an active filter circuit utilizing the mutual conductance of a differential amplifier. Further, the present invention relates to a semiconductor integrated circuit device for signal processing for use in processing a recording signal read from a recording medium (or a magnetic disk) for example and to a technology effectively applied to a disk reading system such as a hard disk drive (or a hard disk memory) utilizing the above-mentioned semiconductor integrated circuit.
Disk reading systems such as a hard disk drive (hereinafter referred to also as a hard disk memory device or an HDD), a floppy disk drive (hereinafter referred to also as an FDD), a CDROM (Compact Disk Read-Only Memory) drive, and an MO (Magneto-Optical) disk drive are used as external storage devices for data processors such as a small workstation and a personal computer. The hard disk drive or a floppy disk drive uses a magnetic disk (or a magnetic recording medium) for information recording medium. The CDROM drive uses an optical disk on which information is stored optically. The MO drive uses an optical disk on which information can be rewritten.
In the hard disk drive or the floppy disk drive, data is written through a magnetic head to a track on the magnetic disk rotated by a spindle motor or read from a track through the magnetic head. A recorded signal read through the magnetic head from the magnetic disk is supplied to a semiconductor integrated circuit for signal processing contained in an external storage device to be removed of an unwanted signal component and a noise component for example. The removal is aimed at those unwanted signal component and noise component whose frequencies exceed a cutoff frequency of a low-pass filter provided in a semiconductor integrated circuit for read channel.
To cope with the ever-increasing processing speed of data processors and the ever-increasing scale of application software, attempts are being made to increase the data recording density (or the surface recording density) of disks and the data transfer rates of hard disk drives, floppy disk drives, CDROM drives, and MO drives.
For a technique to increase the recording density, multi-zone recording is known. In the multi-zone recording, the frequency of a signal to be recorded on a track on the outer periphery side is made relatively greater than the frequency of a signal to be recorded on track on the inner periphery side with the rotational speed (or the angular velocity) of the magnetic disk kept constant. This increases the recording density of the tracks on the outer periphery side, thereby increasing the recording density of the magnetic disk in its entirety. As a result, in the multi-zone recording, the recording density is uniform over all the tracks on the magnetic disk.
Consequently, when reading the data recorded on the magnetic disk by the multi-zone recording, the above-mentioned semiconductor integrated circuit device for signal processing handles the read signal whose frequency changes from low to high. That is, the cutoff frequency of the low-pass filter provided in the semiconductor integrated circuit device is variably controlled according to the frequency of the signal read from the magnetic disk. In other words, when reading the signal from a track on the inner periphery side of the magnetic disk, the cutoff frequency of the low-pass filter is made relatively low, while, when reading the signal from a track on the outer periphery side, the cutoff frequency is made relatively high.
For the low-pass filter provided in the above-mentioned signal-processing semiconductor integrated circuit device, an active filter circuit (or a voltage-controlled filter) utilizing the mutual conductance gm of a differential amplifier is available. For example, such an active filter circuit is described in "Introduction to Analog IC Functional Circuit Design," CQ Publishing Company, p. 151. The active filter circuit comprises a variable conductance circuit having a pair of input transistors whose bases are supplied with an input voltage signal and emitters each provided with a constant current source, a load means commonly attached to the pair of transistors at their collectors via unidirectional devices, a pair of differential transistors whose bases are supplied respectively with the outputs of the collectors of the pair of input transistors, a variable current supply attached to the common emitters of the pair of differential transistors, and current source loads attached respectively to the collectors of the pair of differential transistors so that a half of the current value of the variable current source flows through each of the load, the above-mentioned variable conductance circuit being combined with capacitors. In the active filter circuit thus constituted, the mutual conductance gm of the differential amplifier composed of the pair of differential transistors, current source loads, and the variable current source can be varied by varying, as desired, a value of the current flowing to the pair of differential transistors. Thus, constituting a low-pass filter by using the above-mentioned active filter allows to obtain a desired cutoff frequency.